There are a handful of capitals in hi-sec (ones that were there when they flipped the switch to stop them jumping in) These people have been written to to basically explain if they engage in any combat they will be removed.

TL:DR Moevi is spot on, if you want to know a bit more about the math, read on or if not don’t :D

Wormhole wise, Wormholes have two factors, stable Mass and jumpable Mass, the stable mass relates to the total size of a wormhole, I believe from memory the biggest wormhole into highsec is 5,000,000,000 kg more then ample for a few carrier jumps.

For example a D792 wormhole between a class 5 and hi-sec has a mass limit of 3B Kg and a jumpable limit of 1B.

An average battleship is around 100,000,000 kg certain heavy armour tanks may make this closer to 150,000,000 but you can see well within tollerance. Compare this to a charon which has 960,000,000 kg weight. As you can see much closer this is basically the biggest ship you can get through under the billion limit.

By Contrast a Nidhoggur, which is likely to be the lowest mass carrier due to being a shield tank Minmatar ship has a base mass of 1,014,000,000. Even trying to reduce the mass you are not going to get it down under the bill limit.

This is how CCP has arranged wormhole mechanics to prevent Capitals finding their way into hi-sec.